The RNG works by using a complex mathematical formula to generate a series of random numbers. These numbers are then mapped onto specific symbols on the slot machine's reels. When the player spins the reels, the machine uses the RNG to determine which symbols will appear, and in what order.

The short answer is that slots are programmed for a target percentage in the same way table games are: Odds of the game are set so they will naturally lead to an expected payback percentage.